Change HTML content while block is render in editor

Hello @artf I have one block into block manager like

bm.add("header-with-searchbar",
    {
    label: '<div class="gjs-block-label">Header with searchbar</div>',
    tab: "1",
    category: "Portfolio",
    content: {
    script: function () {
        // I am getting id of new div is added by this block at here.
        const idOfNewDiv = $(this).attr("id");
        // Using below line i am able to edit the DOM, but it will not reflects into editor.getHtml()
        document.getElementById(idOfNewDiv).innerHTML = "<p>Hello</p>";
    },
    content: '<div class="testing"></div>'
    }
}
);

As per above script inside the block, i want to add dynamic HTML content while this block is rendered into the UI.

I know that i don't need to change DOM. But need to change using model, but i don't know how can i change it using mode by id/class.

Can you please help me how can i achieve this?